Fertilizers produced from heterogeneous, phosphorus-rich biowastes are becoming increasingly relevant. Treatment and processing,(combustion, pyrolysis, anaerobic digestion, etc.) increase the diversity of their physicochemical composition even further. We investigated several approaches to characterize P availability from a set of 13 contrasting fertilizers. We tested them directly using standard fertilizer extractions, as well as a continuous, sink-based P extraction (iron bag) method. We also performed Olsen, CAL and diffusive gradients in thin films (DGT) tests on fertilized soil. Standard extractions correlated only weakly, whereas the iron bag method correlated highly (0.73 < R-2 < 0.85) with plant P uptake. Among the tests conducted on fertilized soils, DGT was equivalent or slightly better than Olsen, showing R(2)s of about 0.90 for P uptake and plant growth. Our results suggest that the validity of standard P fertilizer tests needs to be reassessed in the context of increasingly diverse recycling fertilizers. (C) 2017 Elsevier B.V. All rights reserved.